Failure to recognise when an aerosol isn't a droplet 26

[LittleInch]

FacEngrPE dropped this in an obscure post and it reads very well.

Basically the whole epidemiological world though various dieses and viruses were spread by "droplets" which landed on surfaces and then infected people or were sneezed at you.

And a lot of times they are probably right.

But there was a magic 5 micron cut off between droplets and aerosols. Why? Read on

https://www.wired.com/story/the-tee...t-helped-covid-kill/?utm_source=pocket-newtab

So is this a disaster - Well you tell me.
But it goes to show that just because a lot of people write something, it doesn't mean that they originally got the wrong end of the stick and then the error repeats itself until it becomes fact.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LionelHutz]

Since apparently mass can equal volume here at eng-tips.

Are you ready to drop the stupidity of harping on and on about this yet?

Way back in the thread Compositepro posted that for all in intents and purposes, mass = volume.

I simply interpreted this as mass and volume are equivalent and can be used interchangeably. As in, if the volume was reduced by 1/2 then the mass would also reduce by half and by my books that's a pretty good approximation. I expect most others here would agree with this interpretation. Once again, within the context of this subject. It really pisses me off, and should piss everyone else off, when someone goes off about a reason why an answer is wrong when the reason has nothing to do with the subject being discussed.

Still, you decided you didn't like the above mass = volume statement for whatever reason and due to that you "lost it" and began to post a whole bunch of trolling about the V/A ratio being constant, how everyone here is stupid because they accepted the mass = volume statement and also began harping on and on about the mass being the only thing that affects how fast a droplet evaporates, even though that's also wrong. It's way past time you either grow up or go away.

 

[RedSnake]

Can't you two just stopp the cockfighting, please!
And I do not think you need to talk for all others in this forum, Lionel.
If anyone feels insulted or is in disagreement with 1053-44, I am quite sure they can speak for themselves.

You sometimes also have a way of being a bit belittling as in this answer to one of my post.
I wasn't sure if you where addressing me or if it was just a general statment, so I chose to believe in the later.
But couldn't really see the point in you putting it there either and who appointed you engineering god and the one who should decide what all engineers of all professions should or should not know. ​

RedSnake - ya probably the argument is due to that curve. But, you won't find most of that curve within an indoor living area. Yes, droplets aren't perfectly round, but I think you'll find that the V/A ratio of most any shape of droplet changes with volume. We should all know how surface area and evaporation rate are related. anyone who doesn't shouldn't be on an engineering website pretending to be an engineer.

Both of you should take 3 deep breaths and count to 10 before writing anything.
Can't you just agree on disagreeing and stop it?
PLEASE!

/Anna

PS. I see this as a mental degradation due to the pandemic, it seems everyone is getting on everyone's nerves this days.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[LionelHutz]

I'm sorry you felt caught in it Anna. There is a history of trolling type behavior from 1503-44 that I'm sick of seeing. At one point, he was changing his user handle at least daily. There is also a history of calling out a post as wrong and then using the same argument he called out as wrong further into a thread to call out yet another member as being wrong. Here, the argument thing was done, as well as posting snarky responses about how V/A is a constant and the insistent "shutting down" any other response with that mass is the only thing that matters response. Other members have been booted from the site for such behavior but he gets a pass because he's a long term member and is capable of being a valuable contributor.

 

[Alistair_Heaton]

lionel on this one its actually a type of training given to mech eng which revolves round zero energy equations.

Pretty much all systems be it fluid structural electrical and the rest can be dealt with by a mechie using the above technique. You basically get everything to zero out.

I don't know his history in the group but actually his initial injection reminded me of Jim Boyle who actually started as a mathematician then turned into a mechie FEA dude. He was none dimensionless fascist when it came to the physical world. So as most cases the basic form of energy is not geometric they shun anything and go for the base unit for the energy equation.

Please don take this post in anyway as attacking yourself or your Pov. Its just a to say why I understand why geometry stuff linked with a property is shunned by a certain type of engineer.

I was trained the same way to be honest by Jim. But lets face it I was a crap engineer and realised it and went to flying aircraft.

 

[Fischstabchen]

The longer I have been on this forum the more I have grown weary of discussions in non-discipline forums. They are often filled with conclusions that were come to with 15 minute google searches by individuals that have no background to validate what they read. You often see the same kneejerk conclusions that the "uninformed" came to but just dressed up to look more intellectually presentable. In this thread, for example, there is a long debate about droplets but hardly any references of a papers or experiments from the medical community. There are people in academia who have probably spent a substantial amount of their working lives studying transmission. I don't claim to not fall in the same trap sometimes of thinking I can bootstrap myself up on something with a little research but prior to the internet or google, I kind of suspect that there were a lot more people who didn't walk around pretending to be super experts. I understand that this is a forum and these are social discussions and not courtroom proceedings but it grows grating reading through overly vested arguments where neither party is really an expert on the topic on hand. I say this knowing that this forum is stocked full with a lot of very brilliant engineers.

 

[Alistair_Heaton]

Fish you are not wrong with this droplet thing and I really don't pretend to know anything concrete about it.

I do know though that multiple PhD's have been done on the subject and a poo load of cash spent on it by NASA. It is not straight forward otherwise my icing tables would have been set in stone 60 years ago and never changed. Currently they seem to change every 2 years.

 

[IRstuff]

https://www.ht.energy.lth.se/fileadmin/ht/Kurser/MVK160/2012/Holger.pdf[/URL]]The droplet diameters over time for an air velocity of 1m/s and for varying temperatures are shown in Fig. 4.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[FacEngrPE]

I found heat and mass transfer to be a difficult subject filled with dimensionless numbers.

For the case of drops heat transfer, and mass transfer can occur only at the surface. The mass of the drop is mostly just related to the volume.

The relationship between a spheres surface and volume I get from an engineering handbook. While experimenting with drops and rime seems interesting, I will leave that to scientists and engineers that like to write grant applications.

The point of the original posting is - The common language of environmental scientists and the medical community failed the two groups, as they did not realize they were miss communicating, with respect to the behavior of infectious particles. Like many areas of engineering this is one of the cases where good communication is a Life Safety issue.

 

[RedSnake]

Kerfuffle
Sorry! I have never heard that word before it sounded so funny.

After reading the article ones, and sifted through it again I tried to narrow down what it is actually saying, it’s a lot of history and none medical/technical lettering.

The medical world and WHO thought.
• That only droplets under the size of 5 microns could be air born.
• Droplets over the size of 5 microns could only travel max 1,9 m (6 ft) before hitting the ground.

After this work was done the medical world changed it’s view to.
• Droplets of a unknown size could be air born.
• Droplets of a unknown size could travel more then 1,9 m (6 ft) before hitting the ground.

There is no where in this article mentioned what the largest size is for a air born droplet?
There is also no mentioning of what the size of droplets are for those that can travel further then 1,9 m?

That droplets can go further when shouting or singing is logical since the air speed is higher when they come out.
It does not say anything about how much this actually effects the distribution of particularly the SARS-CoV-2 (Covid-19) virus.

The size of the virus is diameter 0,05 to 0,14 microns and it can vary in length with +0,009 to 0,012 microns (I think they mean the spikes here, it was called the tumors).

Another interesting thing with this virus is that, it is a single-stranded RNA virus of genome size ~30kb, and that is the largest known RNA virus genome, almost 3 times longer then what is usual.
If this makes it “heavier” and have a higher density I do not know.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[TugboatEng]

Red, it's very clear in the article.

"According to them, particles bigger than 100 microns sank within seconds. Smaller particles stayed in the air. Randall paused at the curve they’d drawn. To her, it seemed to foreshadow the idea of a droplet-aerosol dichotomy, but one that should have pivoted around 100 microns, not 5. "

Particles under 100 micron are quite able to stay airborne.

However, there was evidence that only much smaller particles were harmful.

"In the report, Langmuir cited a few studies from the 1940s looking at the health hazards of working in mines and factories, which showed the mucus of the nose and throat to be exceptionally good at filtering out particles bigger than 5 microns. The smaller ones, however, could slip deep into the lungs and cause irreversible damage."

A study was done.

"He exposed rabbits to similar doses of the bacteria, pumped into their chambers either as a fine (smaller than 5 microns) or coarse (bigger than 5 microns) mist. The animals that got the fine treatment fell ill, and upon autopsy it was clear their lungs bulged with lesions. The bunnies that received the coarse blast appeared no worse for the wear."

So the small particles are doing the damage. I guess the next question is whether a sub 100 micron particle can remain airborne long enough to evaporate in to a sub 5 micron particle.

 

[RedSnake]

> According to them, particles bigger than 100 microns sank within seconds.

The couple’s calculations made it possible to predict the time it would take a particle of a given size to travel from someone’s mouth to the ground. According to them, particles bigger than 100 microns sank within seconds. Smaller particles stayed in the air. Randall paused at the curve they’d drawn. To her, it seemed to foreshadow the idea of a droplet-aerosol dichotomy, but one that should have pivoted around 100 microns, not 5.

Yes but that was the old studie from 1934 by Wells and his wife, Mildred Weeks Wells that was rejected for the 5 micron theory.
I am not saying it is wrong, I only point out that in this article it is a assumption that 100 microns is the limit, nowhere els is it written in this article that the 1934 results are confirmed.

No one has disputed that small 5 micron droplets can cause damage. (I prefer to call them droplets instead of particles since in my mind a particle is a solid matter without life)

Since this is a virus and they all have different characteristics I am not even sure it needs to get down in to your lungs to be contagious as it is for the tuberculosis virus.
It might be enough that it gets to your mucous membrane in your throat or nose.

/A



“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[LittleInch]

Well I'm glad we've kind of go this thread back onto its original orbit, which wasn't supposed to be an arcane errr discussion about volume, area and mass.

My original point was that the rather arbitrary pivot of 5 microns was something that had become hard wired into the discussion on transmission.

The reality, as usual, is that it is in fact a rather fuzzy grey zone where some particles which can do you harm can float about for a long time even if 10,15 or 25 microns in size. And that was the failure which took a long time to change.

I don't know what other countries are doing, but for along time in the UK the simple message hammered home by politicians (who seem to love three word messages), adverts etc was Hands, Face, Space, as in wash your hands, mask you face and maintain some distance.

Recently that's changed to a rather more unwieldy, but more accurate, Hands, Face, Space and let fresh air in.

My guess is that what is driving new waves is that the virus itself is surviving longer in the air or that you need less numbers of virus particles to make you catch it / get sick.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alistair_Heaton]

I suspect linked back to the fog thing which doesn't form above a certain m/s its air movement related with the open the windows addition.

Which is maybe why there is hardly any cases of people getting it on board aircraft. Everything is rattling through the ducts.

The masks to me were to stop you spraying it, not to protect the wearer from it.

The country's I go to seem to have similar advise. But looking at the numbers it doesn't seem to be very consistent with its effects.

Estonia hardly anyone wears masks and social distancing is also a joke. But apart from a bulge in cases 2 months ago it was pretty stable.

Lithuania everyone is very hot on everything and haven't really had any breaks in the stuff for the last two years and they don't seem to get any benefit.

Latvia in between the two has had another infection profile which is lower. With a similar set of restrictions but more compliance than Estonia.

There is something else going on... Quite what I have no clue.

 

[1503-44]

Driving new waves... the variants are honing their attack. Most new cases are now from variant strains. Its contaigous for reasons I have not yet seen explained. And some vaccines are not so great at defending against them. That's the real danger now. All the people refusing vaccines are allowing chances for the virus to continue mutating and doing so until it finds a new way in.

So. Lionel... its been personal for quite awhile now??? I understand. Troll behavior pattern.
I can't make a mistake with my constants, ratios and proportions, but you can make Einstein, Hawking and Carl Sagan all turn over in their graves. I have been searching for C_p values for volumes of spheres, cubes, pyramids. Cant find any. How do you handle those things.

I also tried to work out F = Sphere x acceleration. For some reason that crashed my Macintosh.
Went to NASA and plugged in E/ c^2. It kept giving me answers in units of Kg. Never once m^3.

I may be bad with ratios and proportions, but at least I can own up to my mistakes and it wasn't ever personal. Guess I have to reevaluate that.

Thanks to the rest that tried to honestly help me see my error and I apologies to those I may have offended. No apologies to those that still may think mass = volume.

 

[RedSnake]

I think we all know now where you two stand, in this.

I have never doubted that you're trying to be serious and I have no opinion on Lionel's claim, as I've not been here that long.

Sometimes I wonder if it's a question of national differences in how one expresses oneself.
The Spanish / Italian temper and the Americans straight forward way of voicing there opinion, now I assume that Lionel is American he might not be.
But that's a completely different discussion, I know.

Please!
Just leave it be.

/Anna

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[moon161]

Right? It was appearing delightfully relevant for a minute.

 

[Eufalconimorph]

This whole thing actually brings up a technique that I feel is often under-used but extremely important for correcting errors: Dimensional Analysis.

It's all too common in math, science, and engineering to write equations with the units for various quantities implicit in the (often single-letter) variable names. That leads to forgetting that they exist, getting constants of proportionality wrong, etc.

(Using TeX instead of sup and sub tags, because I'm being slightly lazy and it's much more convenient to type)...

Take the classic equation:
$E=mc^{2}$

Now let's see what the units are (subscripts for units):
$E_{j}=m_{kg}c_{ms^{-1}}^{2}$


But if we're working in Planck units, that's:
$E_{E_{P}}=m_{m_{P}}1_{l_{P}t_{P}^{-1}}^{2}$


Which simplifies to:
$E_{E_{p}}=m_{m_{P}l_{P}^{2}t_{P}^{-2}}$

Energy is L²MT^-2. So expanding that gives
$E_{m_{P}l_{P}^{2}t_{P}^{-2}}=m_{m_{P}l_{P}^{2}t_{P}^{-2}}$

Thus $E=m$, but ONLY for unit systems where c=hbar=G=1!

Likewise (directly for this discussion), if the density of a substance is 1 (as it very nearly is for water at the temperatures and pressures under discussion in SI units) it's convenient to drop the constant of proportionality and pretend that mass=volume. But doing so can lead to confusion. You could equivalently consider that the molarity is fixed and replace mass with amount of substance, but the constant of proportionality there (6.02214076E23) is obviously not 1 and is thus harder to forget about.

But seriously. If you're doing calculations, include units. It's very helpful for avoiding errors. Sadly most calculators don't allow this. I know the TI-89, TI-92+, and Wolfram Alpha do, but most desktop, phone, and computer calculators don't. Engineers should really do more dimensional analysis. I'd like to build a calculator app that allows it, but it's a lot of work and I haven't found the time to even get started.

 

[LionelHutz]

If telling everyone F-you isn't personal, then what is????

 

[LionelHutz]

I can't make a mistake with my constants, ratios and proportions, but you can make Einstein, Hawking and Carl Sagan all turn over in their graves.

Since you are accusing me of this, do point out in which posts I made such grievous scientific errors.

 

[moon161]

Nobody leaves a urinary olympics with dry feet. Please take it to a new thread.